In recent years, mouse and human iPS cells have been established one after another. Takahashi and Yamanaka induced iPS cells by transferring the Oct3/4, Sox2, Klf4 and c-Myc genes into fibroblasts from a reporter mouse wherein the neomycin resistance gene is knocked-in into the Fbx15 locus, and forcing the cells to express the genes [Takahashi, K. and Yamanaka, S., Cell, 126: 663-676 (2006)]. Okita et al. succeeded in establishing iPS cells (Nanog iPS cells) that show almost the same gene expression and epigenetic modification profiles as those of embryonic stem (ES) cells, by creating a transgenic mouse having the green fluorescent protein (GFP) and puromycin resistance genes integrated into the locus of Nanog, whose expression is more localized in pluripotent cells than the expression of Fbx15, forcing fibroblasts from the mouse to express the above-mentioned four genes, and selecting cells that are puromycin-resistant and GFP-positive cells [Okita, K. et al., Nature, 448: 313-317 (2007)]. Similar results were obtained by other groups [Wernig, M. et al., Nature, 448: 318-324 (2007); Maherali, N. et al., Cell Stem Cell, 1: 55-70 (2007)]. Thereafter, it was revealed that iPS cells could also be produced with 3 factors other than the c-Myc gene [Nakagawa, M. et al., Nat. Biotethnol., 26: 101-106 (2008)].
Furthermore, Takahashi et al. [Takahashi, K. et al., Cell, 131: 861-872 (2007)] succeeded in establishing iPS cells by introducing the same 4 genes as those used in the mouse into human skin fibroblasts. On the other hand, Yu et al. produced human iPS cells using Nanog and Lin28 in place of Klf4 and c-Myc [Yu, J. et al., Science, 318: 1917-1920 (2007)]. Hence, it has been demonstrated that iPS cells comparable to ES cells in terms of pluripotency can be produced in both humans and mice, by transferring defined factors into somatic cells.
Since then, a wide variety of attempts have been made to increase the efficiency of iPS cell establishment, including iPS cells established by transferring TERT and SV40 large T antigen (known as a human cell immortalization genes), along with the four factors Oct3/4, Sox2, Klf4 and c-Myc [Park, I. H. et al., Nature, 451: 141-146 (2008)], iPS cells established with the addition of Nanog and Lin28 to the foregoing four factors [Liao, J. et al., Cell Research, 18: 600-603 (2008)], and iPS cells established with the addition of UTF1 to the foregoing four or three factors other than c-Myc [Zhao, Y. al., Cell Stem Cell, 3: 475-479 (2008)]. However, the situation stands wherein no satisfactory improvement has been achieved.